1. The Field of the Invention
The present invention relates to soft tissue repair surgery, such as rotator cuff repair surgery. More specifically, the present invention relates to a bone anchor used to attach soft tissue to bone using a suture.
2. Related Technology
Soft tissue injuries, especially rotator cuff injuries, can occur from repeated stress or acute trauma. The rotator cuff is a group of muscles and tendons in the shoulder that attach to the humerus bone of the arm. The rotator cuff allows a person to rotate the arm and raise it above the head. A common injury to the rotator cuff occurs when repeated stress or acute trauma causes the rotator cuff to partially or complete tear away from the humerus bone. These and similar types of injuries may require surgery to correctly reattach the soft tissue to the bone.
Various devices have been used to reattach soft tissue to bone. Known methods include staples, wedges, plugs, screws, and sutures alone. Threaded suture anchors, such as bone screws, have recently been developed to provide a particularly firm location where a suture can be anchored to bone. In these systems, a suture is tied between the bone anchor and soft tissue. Providing a firm attachment point for the suture is important because of the relatively strong forces that are experienced in a flexing muscle.
Despite recent advances in bone anchors, existing bone anchors and rotator cuff repairs can fail and have other disadvantages. Typically, a rotator cuff repair fails either because the bone anchor dislodges or the suture tears through the soft tissue. As force is applied to the suture, the suture can cut through the soft tissue like a cheese wire, leaving the rotator cuff detached from the humerus bone. When one suture fails, it can place more stress on the surrounding sutures, thus increasing the likelihood that other sutures will fail in like manner.
Using a greater number of sutures per unit area of soft tissue can minimize suture attachment failure. However, the number of sutures is limited by the space available for inserting bone anchors. Alternatively, additional sutures can be connected to a single bone anchor. Double and triple loading of bone anchors, however, increases the forces applied to the bone anchor and increases the likelihood that the bone anchor will fail by being pulled out of the bone into which it is secured.
Bone anchors can fail for various reasons. One reason is that existing bone anchors are not threaded to the proximal end of the anchor where the anchor meets the surface of the bone in the hard cortical bone region. In existing bone screws, the proximal end is not threaded because the driver tool used to insert the bone anchor fits over a hex shaped protrusion. The hex protrusion cannot extend above the bone surface so the screw is driven into the bone until the protrusion is below the surface. Since the protrusion has no threads, the bone anchor does not engage the bone near the surface, but only the soft cancellous bone beneath the cortical bone layer. This feature of existing bone anchors is very problematic because it prevents a practitioner from placing the threads of the bone anchor in the harder cortical bone, which is near the bone surface.
Some existing anchors engage the sutures above the anchor threads. Such screws have an eyelet formed in the protrusion used to drive the screw. Sutures are looped in the eyelet and fed through the driver tool. Because the attachment point for the sutures is above the threads, the threaded portion of the bone anchor experiences a high torque moment when a force is applied by the suture to the attachment point. Consequently, existing bone anchors are prone to experience a certain degree of movement within the soft bone layer. In some cases, the bone anchor can retreat from the bone and rub against soft tissues overlaying the bone anchor. This may result in the irritation or damage of such tissues.
Recently, bone screws have been used that place the attachment point for the sutures within the body of the screw, specifically within the bore of the screw. In these screws an attachment site within the bore is created using a small piece of suture. The ends of the suture are fed through holes near the proximal end of the screw to form a small suture loop within the bore. Knots are tied in the end of the suture to prevent the ends from passing back though the holes. The suture loop within the bore provides an anchoring point for threading sutures used to secure tissue in a surgical procedure.
One problem with these screws is that the knots that hold the suture loop can come untied and/or break, which releases the sutures anchored to the suture loop. This problem is particularly difficult to fix once the screw has been placed in a patient because it would require the surgeon to remove the screw from the patient. An additional problem with using suture loops as an attachment mechanism is that the suture material can fray as the anchored sutures slide on it. Furthermore, when loading multiple sutures on the suture loop, the suture loop can flex and cause the sutures to bunch together. When a practitioner is using a suture the practitioner needs the suture to freely slide in the bone anchor. The friction and/or pinching that a suture loop can cause between lengths of suture can cause suture capture, which is undesirable.
Therefore, what is needed is a bone anchor that better engages its surrounding bone tissue and allows greater forces to be applied to the bone anchor without becoming dislodged or failing. In addition, a bone anchor is needed that can reduce suture capture.